Performance per gallon is a tricky thing to measure, but the 2014 Porsche 911 scores well. On the actual gas mileage front, it doesn't do too shabbily, either.
The base Carrera with PDK is the most efficient offering, at 20 mpg city and 28 mpg highway--very respectable figures for any 350-horsepower daily driver. Adding 50 horsepower and all-wheel drive, as in the Carrera 4S, with the seven-speed manual transmission, reduces the score just 2 mpg to 18/26 mpg.
Most of the other 991-based models fall somewhere between those bookends, except for the Turbo and Turbo S, which return an EPA score of 17 mpg city and 24 mpg highway, and the GT3, which scores just 15 mpg city and 20 mpg highway.